BGA Overview
BGA is a type of chip package, short for Ball Grid Array in English. The package pins are ball grid arrays at the bottom of the package, and the pins are spherical and arranged in a grid-like pattern, hence the name BGA.
Many motherboard control chips use this type of packaging technology, and the materials are mostly ceramic. Memory packaged with BGA technology can increase the memory capacity by two to three times without changing the volume. Compared with TSOP, BGA has a smaller volume, better heat dissipation, and electrical performance.
BGA Package Pad Routing Design
1. Routing between BGA pads
During the design, the BGA pad spacing is less than 10mil, and routing is not allowed between two BGAs, because the line width spacing of the routing exceeds the production process capability. If routing is to be done, the BGA pad can only be reduced. When making the production draft, ensuring that the spacing is sufficient will cut the BGA pad. The pad is cut into a special shape, which may cause inaccurate welding position in subsequent welding.
2. Filling the via in the pad with resin plugging
When the pad spacing of the BGA package is small and the wire cannot be routed, a via in the pad needs to be designed, that is, the hole is punched on the pad and the wire is routed from the inner layer or the bottom layer. At this time, the via in the pad needs to be filled with resin plugging and electroplating. If the via in the pad does not adopt the resin plugging process, it will lead to poor welding during welding, because there is a hole in the middle of the pad and the welding area is small, and tin will leak from the hole.
3. BGA area via plugging
The vias in the BGA pad area generally need to be plugged. For the sample, considering the cost and production difficulty, the basic vias are covered with oil. The plugging method is ink plugging. The advantage of plugging is to prevent foreign matter in the hole or protect the service life of the via. In addition, when the SMT patch is reflowed, the via tin will cause a short circuit on the other side.
4. Via in the pad, HDI design
For BGA chips with relatively small pin spacing, when the pin pad cannot be routed due to the process, it is recommended to directly design a via in the pad. For example, the BGA chip of the mobile phone board is relatively small, with many pins and small pin spacing, so it is impossible to route the wires from the middle of the pins. Only the HDI blind buried hole wiring method can be used to design the PCB. The BGA pad is punched with a hole in the plate, the inner layer is punched with a buried hole, and the inner layer is wired and connected.
BGA Welding Process Quality
1. Printing solder paste
The purpose of solder paste printing is to evenly apply an appropriate amount of solder paste to the pads of the PCB to ensure that the patch components and the corresponding pads of the PCB are reflow soldered to achieve good electrical connection and sufficient mechanical strength. To print solder paste, we need to make a steel mesh. The solder paste passes through the corresponding openings of each pad on the steel mesh, and the tin is evenly coated on each pad under the action of the scraper to achieve good welding.
2. Device placement
Device placement is patching, which is to use a placement machine to accurately place chip components on the corresponding position of the PCB surface printed with solder paste or patch glue. High-speed placement machines are suitable for mounting small and large components: such as capacitors, resistors, etc., and can also mount some IC components. General-purpose placement machines are suitable for mounting heterogeneous or high-precision components: such as QFP, BGA, SOT, SOP, PLCC, etc.
3. Reflow soldering
Reflow soldering is to melt the solder paste on the circuit board pad to achieve mechanical and electrical connection between the surface-mounted component soldering end and the PCB pad to form an electrical circuit. Reflow soldering is a key process in SMT production. Reasonable temperature curve setting is the key to ensure the quality of reflow soldering. Inappropriate temperature curve will cause welding defects such as incomplete soldering, cold soldering, component warping, excessive solder balls, etc. on the PCB board, affecting product quality.
4. X-Ray inspection
X-Ray can check almost all process defects. Through the perspective characteristics of X-Ray, the shape of the solder joint can be checked and compared with the standard shape in the computer library to judge the quality of the solder joint. This is particularly useful for solder joint inspection of BGA and DCA components. The role of X-Ray inspection is irreplaceable, as it does not require test molds. However, the disadvantage is that the cost of X-Ray inspection is currently quite expensive.
Reasons for Poor BGA Welding
1. Unprocessed BGA pad holes
There are holes on the pads of BGA welding. During the welding process, the solder balls may be lost together with the solder. Due to the lack of a proper resistance welding process in PCB production, the solder and solder balls can escape through the holes near the welding board, resulting in the loss of solder balls.
2. Different pad sizes
The different sizes of BGA solder pads can affect the quality yield of the welding process. The lead-out wire of the BGA pad should not exceed 50% of the pad diameter, and the lead-out wire of the power pad should not be less than 0.1mm. It should also be thickened to prevent the welding pad from deforming. Additionally, the welding blocking window should not be larger than 0.05mm, and the opening on the copper surface should match the size of the circuit PAD. Otherwise, the BGA pads will be made in different sizes, which can cause issues during the welding process.
wonderfulpcb DFM Services About BGA Chip Welding Solution
1. Packaged Pad-in-Pad Hole
wonderfulpcb DFM Services one-click analysis detects whether there is a pad-in-pad hole in the design file, and prompts the design engineer if the pad-in-pad hole needs to be modified. The design of pad-in-pad holes is often avoided due to the high manufacturing cost. If the pad-in-pad hole can be changed to an ordinary hole, the cost of the product can be reduced. Additionally, the system alerts the manufacturing board factory that the pad-in-pad hole design needs to be filled with resin, and that the pad-in-pad hole production process must be used.
2. Pad-to-pin ratio
wonderfulpcb DFM Services assembly analysis detects the size ratio of the BGA pad in the design file relative to the actual device pin. If the pad diameter is less than 20% of the BGA pin, it may lead to poor welding. Conversely, if it is greater than 25%, the wiring space becomes too small. In such cases, the design engineer needs to adjust the ratio of the pad to the BGA pin diameter.
wonderfulpcb DFM Services provides BGA pad solderability solutions, helping users review the solderability of BGA design files before production. This helps avoid solderability problems during assembly, and ensures BGA chips meet solderability quality yield standards.
BGA Welding Process Quality
1. Printing solder paste
The purpose of solder paste printing is to evenly apply an appropriate amount of solder paste to the pads of the PCB to ensure that the patch components and the corresponding pads of the PCB are reflow soldered to achieve good electrical connection and sufficient mechanical strength. To print solder paste, a steel mesh is used. The solder paste passes through the corresponding openings of each pad on the steel mesh, and the tin is evenly coated on each pad under the action of the scraper to achieve good welding.
2. Device placement
Device placement is patching, which involves using a placement machine to accurately place chip components on the corresponding position of the PCB surface, which is printed with solder paste or patch glue. High-speed placement machines are suitable for mounting small and large components, such as capacitors, resistors, and some IC components. General-purpose placement machines are suitable for mounting heterogeneous or high-precision components, such as QFP, BGA, SOT, SOP, PLCC, etc.
